Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for updating and/or optimizing a driving signal of a seismic source.
Discussion of the Background
Reflection seismology is a method of geophysical exploration to determine the properties of a portion of a subsurface layer in the earth, which is information especially helpful in the oil and gas industry. Marine reflection seismology is based on the use of a controlled source that sends energy waves into the earth. By measuring the time it takes for the reflections to come back to one or more receivers, it is possible to estimate the depth and/or composition of the features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
For marine applications, sources are typically impulsive, e.g., compressed air stored in an air gun is suddenly allowed to expand, thus forming an oscillating air bubble that generates seismic energy. An air gun produces a great amount of acoustic energy over a short time. Such a source is towed by a vessel either at the water surface or at a certain depth. The acoustic waves from the air gun propagate in all directions. A typical frequency range of the emitted acoustic waves is between 6 and 300 Hz. However, the frequency content of impulsive sources is not fully controllable, and different sources are selected depending on a particular survey's needs. In addition, the use of impulsive sources can pose certain safety and environmental concerns. A further disadvantage of air guns is the inability to adjust frequency spectrum or frequency content while the air guns are towed in water and activated.
Another class of sources that may overcome some of the air guns' limitations are vibratory sources. Vibratory sources, including hydraulically, electrically, or pneumatically powered sources and sources employing piezoelectric or magnetostrictive material, have been used in marine operations.
A vibratory source generates a long tone with a varying frequency, i.e., a frequency sweep when a pilot signal is applied to the source. Typically, the pilot signal is stored in a vibrator controller memory. Upon receiving a start command from the seismic acquisition management system, the vibrator controller generates the pilot signal and uses it as a reference signal input to a closed-loop feedback control system whose function is to slave, for example, the motion of a moving part, like an acoustic piston to the pilot signal. The motion of the piston displaces a volume of water, which generates a corresponding seismic wave. The instantaneous pressure resulting from the movement of one or more pistons corresponding to plural marine vibrators may be lower than that of an air gun array, but the total acoustic energy the marine vibrator transmits may be similar to the energy of the air gun array due to the extended duration of the vibratory source's signal. However, such sources need a frequency sweep to achieve the required energy. The term frequency sweep, can refer to either a swept sine wave or a band-limited pseudorandom signal, designed to have a non-zero amplitude spectrum over a range of frequencies using a continuous or semi-continuous signal that is used as a pilot signal to excite the seismic source over some time duration.
U.S. Patent Application Publication No. 20100118647A1, entitled, “Method for optimizing energy output from a seismic vibrator array,” the entire disclosure of which is incorporated herein by reference, discloses two flextensional vibrators (low frequency and high frequency) activated by electro-mechanical actuators and emitting seismic energy at two different depths during a frequency sweep. The vibrators are driven by swept frequency signals, each having a different selected frequency response.
A non-linear frequency sweep is also described in U.S. Pat. No. 6,942,059B2, entitled, “Composite bandwidth marine vibroseis array,” the entire content of which is incorporated herein by reference. This document discloses a method for seismic marine survey using vibrator sources, each of them placed at different depths. The vibrator sources show a level of seismic energy comparable to an air gun array (single depth) by dividing the seismic bandwidth over a plurality of different bandwidths. Each bandwidth is generated by a vibrator array using a non-linear sweep in order to maximize output energy.
A more sophisticated sweep design method applied to hydraulic seismic vibrators is disclosed in U.S. Pat. No. 8,274,862, entitled, “System and method for determining a frequency sweep for seismic analysis,” (assigned to the assignee of the present application), the entire content of which is incorporated herein by reference. This method takes into account not only the plate stroke limit, but also other land vibrator constraints, e.g., the pump flow limit and the servo valve flow limit. U.S. patent application Ser. No. 13/677,661 entitled “Device and Method for Continuous Data Acquisition,” (assigned to the assignee of the present application), the entire content of which is incorporated herein by reference, teaches a method for generating a marine vibrator pilot signal that is a band-limited pseudorandom signal so that a target amplitude spectrum can be achieved given certain system operating constraints.
However, no existing method is flexible enough to update and/or optimize the frequency sweep as various field conditions dictate. Thus, there is a need to provide a method for monitoring one or more features related to the source and/or its environment and updating the frequency sweep based on results of the monitoring process.